A: 1. First, check if the relay is functioning correctly.
① Check the relay terminals according to the service manual. Under power-off conditions, it should be in an open circuit state; otherwise, it may be stuck together.
② Power on the UPS. With the rectifier starting but the inverter not yet fully running, under normal circumstances, there should be inverter voltage but no output voltage or a very small value. Otherwise, a short circuit exists.
2. Check for other accompanying alarms. Check if the inverter IGBT and bypass SCR are functioning correctly.
3. Check if the signal cables between the output board and the control board are properly connected. Try unplugging and replugging them.
4. The voltage sampling or operational amplifier function on the control board may be faulty.

Solution: 
1. It is recommended to replace the output board first. 
2.If the alarm persists, it is recommended to replace the signal cables and the control board.
(When replacing the control board, take precautions against static electricity and apply hot melt glue to the signal cable connections.)